Cysteine and cystine prodrugs to treat schizophrenia and reduce drug cravings

ABSTRACT

The present invention provides cysteine and cystine prodrugs for the treatment of schizophrenia and drug addiction. The invention further encompasses pharmaceutical compositions containing prodrugs and methods of using the prodrugs and compositions for treatment of schizophrenia and drug addiction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/367,867, filed Feb. 9, 2009, which claims the benefit ofU.S. Provisional Application Ser. No. 61/026,874, filed Feb. 7, 2008.All applications are incorporated herein by reference as if set forth intheir entirety.

FIELD OF THE INVENTION

This invention relates generally to the treatment of schizophrenia anddrug addiction. More particularly, the present invention is directed tocysteine and cystine prodrugs useful as antipsychotic medications in thetreatment of schizophrenia. As well, the respective prodrugs areapplicable for reducing drug cravings in drug addicted individuals.

BACKGROUND OF THE INVENTION

Schizophrenia is a debilitating disorder afflicting 1% of the world'spopulation. The development of effective medications to treatschizophrenia is reliant on advances in characterizing the underlyingpathophysiology. Chlorpromazine and other phenothiazines are consideredfirst generation antipsychotics (termed “typical antipsychotics”) usefulin the treatment of schizophrenia. However, the antipsychotic efficacyof phenothiazines was, in fact, serendipitously discovered. These drugswere initially used for their antihistaminergic properties and later fortheir potential anesthetic effects during surgery. Hamon and colleaguesextended the use of phenothiazines to psychiatric patients and quicklyuncovered the antipsychotic properties of these compounds; shortlythereafter, the pharmacologic characteristic of dopamine receptorblockade was linked to the antipsychotic action of chlorpromazine(Thorazine). This led to the development of additional dopamine receptorantagonists, including haloperidol (Haldol). For nearly fifty years,dopamine antagonists were the standard treatment for schizophrenia eventhough these drugs induce severe side effects ranging from Parkinson'sdisease-like motor impairments to sexual dysfunction and are onlyeffective in treating the positive symptoms of schizophrenia.

In the 1970's, clozapine became the first “atypical psychotic” or 2ndgeneration antipsychotic agent introduced. Clinical trials have shownthat clozapine produces fewer motor side effects and exhibits improvedefficacy against positive and negative symptoms relative to 1stgeneration compounds. However, clozapine was briefly withdrawn from themarket because of the potential to produce severe agranulocytosis, apotentially fatal side effect requiring patients to undergo routine,costly hematological monitoring. As a result, clozapine is only approvedfor treatment-resistant schizophrenia. Although also a dopamine receptorantagonist, the therapeutic site of action for clozapine is thought toinvolve blockade of serotonin receptors. This led to the generation ofother serotonin receptor antagonists in the 1990's with the goal ofimproving the safety profile of clozapine.

The growth potential for novel antipsychotics was revealed following theintroduction of risperidone in 1994; within two years risperidoneovertook haloperidol in the number of prescriptions written byphysicians. While it was generally assumed that the newer 2nd generationantipsychotics also exhibited the favorable efficacy profile produced byclozapine, the clinical data was ambiguous. As a result, the NIHrecently funded a large, lengthy, and expensive clinical trial toexamine this assumption. The results of the Clinical AntipsychoticTrials of Intervention Effectiveness (CATIE), recently released,indicate that there is no benefit to the newer 2nd generation compounds.Specifically, 1st and 2nd generation drugs did not differ in theincidence of severe motor side-effects nor were 2nd generation agentsfound to be more effective than 1st generation antipsychotics. In theCATIE trial, 74% of the patients discontinued treatment prior tocompleting the 18 month trial, in part due to a lack of efficacy andintolerability of the treatment regimen.

As can be appreciated from the foregoing, there exists a pressing needand considerable market potential for novel antipsychotic agents. Ofcourse, the development of effective antipsychotic agents will befacilitated by a thorough understanding of pathophysiologies underlyingthe neurological disorders.

SUMMARY OF THE INVENTION

The present invention is based on the inventors' success in identifyingprodrugs of cysteine and cystine with utility as antipsychotic andaddiction reducing agents. Accordingly, the invention provides acysteine prodrug having the structure:

wherein: R¹ and R² are independently selected from OH, ═O, or a branchedor straight chain C₁ to C₅ alkoxyl group, with the caveat that when ═Ois selected the nitrogen atom adjacent the carbonyl group thusly formedbears a H and a single bond joins the adjacent nitrogen to said carbonylgroup; R³ is H, a branched or straight chain C₁ to C₅ alkyl, anitrobenzenesulfonyl, a trityl, an aryl thio, an aryl, an alkylthio, anacyl, a benzoyl, a thio acyl, a thio benzoyl, or a benzyl group; and R⁴is selected from the side chain groups of the natural L-amino acids cys,gly, phe, pro, val, ser, arg, asp, asn, glu, gln, ala, his, ile, leu,lys, met, thr, trp, tyr, or D-isomers thereof, with the caveat that whenR⁴ is the side chain group of the natural L-amino acid gly, R¹ and R²are not both selected to be ═O; or a cystine dimer of said prodrughaving the structure:

wherein: R¹, R^(2,) R⁵ and R⁶ are independently selected from OH, ═O, ora branched or straight chain C₁ to C₅ alkoxyl group, with the caveatthat when ═O is selected the nitrogen atom adjacent the carbonyl groupthusly formed bears a H and a single bond joins the adjacent nitrogen tosaid carbonyl group; and R⁴ and R⁷ are independently selected from theside chain groups of the natural L-amino acids cys, gly, phe, pro, val,ser, arg, asp, asn, glu, gln, ala, his, ile, leu, lys, met, thr, trp,tyr, or D-isomers thereof, with the caveat that when R⁴ and R⁷ are boththe side chain group of the natural L-amino acid gly, R¹, R^(2,) R⁵ andR⁶ shall not all be selected to be ═O.

In certain preferred embodiments, the cysteine prodrug according to theinvention has the structure:

The cysteine prodrug may alternatively be provided in the form of acystine dimer. Certain preferred cystine dimers according to theinvention have the structure: the form of the cystine dimer having thestructure:

The invention provides synthetic routes for the synthesis of cystinedimers having identical R⁴ and R⁷ groups or, alternatively, mixed ornon-identical R⁴ and R⁷ groups.

In certain cysteine prodrugs or cystine dimers of the invention, atleast one R⁴ and R⁷ group is a cys and the reactive moiety is furtherprotected by a branched or straight chain C₁ to C₅ alkyl, anitrobenzenesulfonyl, a trityl, an aryl thio, an aryl, an alkylthio, anacyl, a benzoyl, a thio acyl, a thio benzoyl, or a benzyl group.

In another aspect, the present invention provides a method of reducingschizophrenia in a subject. Such a method includes steps ofadministering to the subject an effective amount of a cysteine prodrugor cystine dimer thereof according to the invention, wherebyschizophrenia is reduced in the subject. Administration is preferablyaccomplished by oral delivery.

In yet another aspect, the invention provides a method of reducing drugcraving in a subject. Such a method includes steps of administering tothe subject an effective amount of a cysteine prodrug or cystine dimerof the invention, whereby drug craving is reduced in the subject. Again,administration is preferably via the oral route.

Of course, the present invention encompasses pharmaceutical compositionsincluding a cysteine prodrug or cystine dimer according to the inventionin combination with at least a pharmaceutically-acceptable carrier. Theinvention further contemplates methods for the manufacture of such apharmaceutical composition for the reduction of schizophrenia and/ordrug craving in a subject.

A further aspect of the invention encompasses protected cysteine analogshaving the structure:

or a cystine dimer of the protected cysteine analog having thestructure:

wherein R¹ through R⁶ are independently selected from a branched orstraight chain C₁ to C₅ alkyl, a phenyl, or a benzyl group.

Preferable protected cysteine analogs according to the invention havethe structure:

Alternatively, protected cysteine analogs may be provided in the form ofthe corresponding cystine dimers. Certain preferred cystine dimers havethe structures:

Related to the protected cysteine analogs, the invention furtherprovides a method of reducing schizophrenia in a subject byadministering to a subject an effective amount of a protected cysteineanalog or cystine dimer thereof according to the invention, wherebyschizophrenia is reduced in said subject. Administration is preferablyvia the oral route.

The invention is also directed to protected cysteine analogs or cystinedimers thereof having any one of the structures described and claimedherein. Such analogs are useful in methods of reducing schizophrenia orreducing drug cravings in a subject comprising administering to thesubject an effective amount of the protected cysteine analog or cystinedimer.

The invention further encompasses pharmaceutical compositions containinga protected analog or dimer thereof in combination with apharmaceutically-acceptable carrier. Methods offormulating/manufacturing such pharmaceutical compositions for thetreatment of schizophrenia or for reducing drug craving in a subject arealso within the invention's scope.

Other objects, features and advantages of the present invention willbecome apparent after review of the specification, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts diketopiperazine targets based on various natural L-aminoacids and D-isomers thereof.

FIG. 2 illustrates general formulas for monomer and dimer prodrugs andprecursors thereof.

FIG. 3 provides general formulas for various protected cysteine analogs.

FIG. 4 illustrates percent inhibition of a startle response elicited bya loud auditory stimulus (50 dB above background) when preceded by amild auditory stimulus (2-15 dB above background) in rats treated withpcp (0-2.0 mg/kg, N=9-60/group). * from every other group at respectiveprepulse intensity, Fisher LSD p<0.05

FIG. 5 displays the impact of N-acetyl cysteine on sensorimotor gatingdeficits produced by phencyclidine administered orally (left) ordirectly into the prefrontal cortex (right), which is likely thetherapeutic site of action for cysteine prodrugs. * from every pcp onlygroup at respective prepulse intensity, Fisher LSD p<0.05

FIG. 6 illustrates the efficacy of exemplary compounds from Scheme 1relative to N-acetyl cysteine in reversing PCP-induced deficits insensorimotor gating in rats. * from every pcp only group at respectiveprepulse intensity, +NAC 30 group, Fisher LSD p<0.05

FIG. 7 shows the efficacy of exemplary compounds from Scheme 2 relativeto N-acetyl cysteine in reversing PCP-induced deficits in sensorimotorgating in rats. * from every pcp only group at respective prepulseintensity, +NAC 30 group, Fisher LSD p<0.05

FIG. 8 illustrates the efficacy of exemplary compounds from Scheme 3relative to N-acetyl cysteine in reversing PCP-induced deficits insensorimotor gating in rats. * from every pcp only group at respectiveprepulse intensity, +NAC 30 group, Fisher LSD p<0.05

FIG. 9 shows the efficacy of exemplary compounds from Scheme 4 relativeto N-acetyl cysteine in reversing PCP-induced deficits in sensorimotorgating in rats. * from every pcp only group at respective prepulseintensity, +NAC 30 group, Fisher LSD p<0.05

FIG. 10 illustrates the efficacy of compound from scheme 5 relative toN-acetyl cysteine in reversing PCP-induced deficits in sensorimotorgating in rats. * from every pcp only group at respective prepulseintensity, +NAC 30 group, Fisher LSD p<0.05

FIG. 11 provides a bar graph illustrating that N-acetylcysteine (IP) iseffective in producing a significant reduction in cocaine-inducedreinstatement at the doses of 30 and 60 mg/kg.

FIG. 12 depicts a bar graph illustrating that N-acetylcysteine is lesseffective when given orally. Further, administration of 1 mg/kg ofCompound 5a-D (Scheme 1) was sufficient to block cocaine-inducedreinstatement, an effect that was comparable to 30 mg/kg NAC.

DETAILED DESCRIPTION OF THE INVENTION

Before the present materials and methods are described, it is understoodthat this invention is not limited to the particular methodology,protocols, materials, and reagents described, as these may vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto limit the scope of the present invention which will be limited onlyby the appended claims.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural reference unless thecontext clearly dictates otherwise. As well, the terms “a” (or “an”),“one or more” and “at least one” can be used interchangeably herein. Itis also to be noted that the terms “comprising”, “including”, and“having” can be used interchangeably.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications and patentsspecifically mentioned herein are incorporated by reference for allpurposes including describing and disclosing the chemicals, cell lines,vectors, animals, instruments, statistical analysis and methodologieswhich are reported in the publications which might be used in connectionwith the invention. All references cited in this specification are to betaken as indicative of the level of skill in the art. Nothing herein isto be construed as an admission that the invention is not entitled toantedate such disclosure by virtue of prior invention.

The term “lower alkyl group(s)” as used herein indicates a linear,branched or cyclic alkyl group(s) having 1 to 6 carbon atoms. Theyinclude, for example, methyl group, ethyl group, n-propyl group, n-butylgroup, n-pentyl group, n-hexyl group, isopropyl group, isobutyl group,sec-butyl group, tert-butyl group, isopentyl group, tert-pentyl group,neopentyl group, 2-pentyl group, 3-pentyl group, 3-hexyl group, 2-hexylgroup, cyclopropyl group, cyclobutyl group, cyclopentyl group andcyclohexyl group. In them, methyl group, ethyl group, etc. arepreferred.

The term “aryl group(s)” as used herein indicates a monocyclic orbicyclic aromatic substituent(s) composed of 5 to 12 carbon atoms, suchas phenyl group, indenyl group, naphthyl group and fluorenyl group. Inthem, phenyl group is preferred. The term “arylthio group” indicates amonocyclic or bicyclic aromatic substituent(s) composed of 5 to 12carbon atoms and further including a thio moiety.

The term “alkylthio group(s)” as used herein indicates an alkylthiogroup(s) having a linear, branched or cyclic alkyl group having 1 to 12carbon atoms, preferably 1 to 5 carbon atoms, such as methylthio group,ethylthio group, n-propylthio group, isopropylthio group, n-butylthiogroup, isobutylthio group, sec-butylthio group, tert-butylthio group,cyclopropylthio group, cyclobutylthio group, cyclopentylthio group andcyclobutylthio group.

The term “acyl group(s)” as used herein indicates a formyl group, anacyl group(s) having a linear, branched or cyclic alkyl group having 1to 6 carbon atoms, acyl group(s) having a linear, branched or cyclicalkenyl group having 1 to 6 carbon atoms, acyl group(s) having a linear,branched or cyclic alkynyl group having 1 to 6 carbon atoms or acylgroup(s) having an aryl group which may be substituted, such as formylgroup, acetyl group, propionyl group, butyryl group, isobutyryl group,valeryl group, isovaleryl group, pivaloyl group, hexanoyl group,acryloyl group, methacryloyl group, crotonoyl group, isocrotonoyl group,benzoyl group and naphthoyl group. Acyl groups having a heterocyclicring can also be used, for example, furanyl carbonyl group, thienylcarbonyl group, isoxazolyl carbonyl group and thiazolyl carbonyl group.

The term “thio acyl group(s)” as used herein indicates a thio acylgroup(s) having a linear, branched or cyclic alkyl group having 1 to 6carbon atoms, thio acyl group(s) having a linear, branched or cyclicalkenyl group having 1 to 6 carbon atoms, thio acyl group(s) having alinear, branched or cyclic alkynyl group having 1 to 6 carbon atoms orthio acyl group(s) having an aryl group which may be substituted, suchas formyl group, acetyl group, propionyl group, butyryl group,isobutyryl group, valeryl group, isovaleryl group, pivaloyl group,hexanoyl group, acryloyl group, methacryloyl group, crotonoyl group,isocrotonoyl group, benzoyl group and naphthoyl group. Thio acyl groupsmay be incorporated in a heterocyclic ring, for example, thienylcarbonyl group and thiazolyl carbonyl group.

The term “amino acid” refers to an organic acid containing an aminogroup. The term includes naturally occurring amino acids (“natural aminoacids”) such as alanine, valine, leucine, isoleucine, proline,phenylalanine, tryptophan, methionine, glycine, serine, threonine,cysteine, asparagine, glutamine, tyrosine, histidine, lysine, arginine,aspartic acid, and glutamic acid. Amino acids can be pure L or D isomersor mixtures of L and D isomers.

“Prodrugs” refers to compounds, including monomers and dimers of thecompounds of the invention, which have cleavable groups and become underphysiological conditions compounds which are pharmaceutically active invivo.

“Subject” includes humans. The terms “human,” “patient” and “subject”are used interchangeably herein.

“Therapeutically effective amount” means the amount of a compound that,when administered to a subject for treating a disease or disorder, issufficient to effect such treatment for the disease or disorder. The“therapeutically effective amount” can vary depending on the compound,the disease or disorder and its severity, and the age, weight, etc., ofthe subject to be treated.

“Treating” or “treatment” of any disease or disorder refers, in oneembodiment, to ameliorating the disease or disorder (i.e., arresting orreducing the development of the disease or at least one of the clinicalsymptoms thereof). In another embodiment “treating” or “treatment”refers to ameliorating at least one physical parameter, which may not bediscernible by the subject. In yet another embodiment, “treating” or“treatment” refers to modulating the disease or disorder, eitherphysically, (e.g., stabilization of a discernible symptom),physiologically, (e.g., stabilization of a physical parameter), or both.In yet another embodiment, “treating” or “treatment” refers to delayingthe onset of the disease or disorder, or even preventing the same.

The present inventors have recently identified the cystine-glutamateantiporter as a highly novel cellular process that likely contributes tothe pathology underlying schizophrenia. Importantly, the inventors havecollected the first data set indicating that cysteine prodrugs, used toincrease the activity of cystine-glutamate antiporters, block cognitivedeficits and social withdrawal in the preclinical phencyclidine model ofschizophrenia. Unlike existing medications, cysteine prodrugs appear toexert antipsychotic properties, in part, by reversing pathologyunderlying the disease.

While no one theory or mechanism of pharmacological effect is adoptedherein, cysteine prodrugs appear to restore diminished signaling toglutamate receptors and diminished glutathione levels observed inschizophrenics. A depleted glutathione level can lead to increasedoxidative stress, and impaired cystine-glutamate antiporter activity,glutamate neurotransmission, synaptic connection, and gene expression,all of which are observed in schizophrenia.

As a related matter, as made evident by the inventors' findings,impaired cystine-glutamate antiporter activity and faulty glutamateneurotransmission bear on the issue of uncontrolled drug use, i.e., drugaddiction. Uncontrolled drug use and heightened susceptibility torelapse are defining features of addiction that contribute to thetransition in drug consumption from a recreational to a compulsivepattern. Long-term plasticity resulting in augmented excitatoryneurotransmission within corticostriatal pathways in response to drugsof abuse have been implicated in addiction. Human cocaine abusersexposed to craving-inducing stimuli exhibit increased activation ofexcitatory circuits originating in cortical regions, including orbitaland prefrontal cortex, and projecting to the ventral striatum; further,the degree of activation of corticostriatal pathways correlates withcraving in humans.

Preclinical data also indicate the existence of drug-induced plasticityleading to activation of corticostriatal pathways. Activation of thesecircuits results in heightened extracellular glutamate in the nucleusaccumbens and stimulation of ionotropic glutamate receptors, both ofwhich are necessary for cocaine primed reinstatement. Further, thedorsomedial prefrontal cortex has been shown to be necessary forreinstatement produced by exposure to drug-paired cues using thecontextual reinstatement paradigm and in response to electrical footshock. As a result, identification of cellular mechanisms capable ofregulating synaptic glutamate represent targets in the treatment ofaddiction.

Increased excitatory neurotransmission in the nucleus accumbens mayarise, in part, by diminished activity of cystine-glutamate antiporters.The recent data collected by the present inventors illustrates thatglutamate released from these antiporters provides endogenous tonicstimulation to group II or 2/3 metabotropic glutamate receptors (mGluRs)and thereby regulates synaptic glutamate and dopamine release. Thus,altered glutamate signaling could arise as a consequence of decreasedcystine-glutamate exchange. Repeated cocaine administration has beenshown to blunt the activity of cystine-glutamate exchange, which likelycontributes to a sequence of events, including diminished group II mGluRautoregulation and increased excitatory neurotransmission in the nucleusaccumbens.

Cysteine prodrugs, such as N-acetylcysteine (“NAC”), are used to drivecystine-glutamate exchange by apparently elevating extracellular cystinelevels, thereby creating a steep cystine concentration gradient.Preclinical studies have shown N-acetylcysteine to be effective inblocking compulsive drug-seeking in rodents. Further, extant clinicaldata also show a reduction in cocaine use and craving in cocaine abusersreceiving NAC. Unfortunately, the full clinical efficacy of targetingcystine-glutamate exchange may be unrealized when utilizing NAC due toextensive first-pass metabolism and limited passive transport of thisdrug across the blood-brain barrier. The prodrugs described and claimedherein will not be significantly eliminated by the liver and willreadily pass the blood-brain barrier. Cysteine is the reduced form ofcystine and is readily oxidized in vivo to cystine, thus elevatingeither cysteine or cystine is believed to increase cystine-glutamateexchange.

The cysteine prodrug NAC has been previously shown to have a favorablesafety/tolerability profile in human subjects. In fact, NAC has beenused for decades in humans for other indications (e.g., as a mucolytic,acetaminophen toxicity) and as an experimental treatment (HIV, cancer)without producing severe adverse effects. However, NAC undergoesextensive first pass metabolism requiring the usage of high doses thatlimit the utility of the drug and, potentially, increase the chances ofside effects due to the buildup of metabolized by-products. The chemicalentities presently disclosed and claimed herein are designed tosubstantially avoid the problem of first pass metabolism and thereforeexhibit increased efficacy as compared to prior cysteine prodrugs.

The preferred synthetic route to provide cysteine prodrugs according tothe invention will now be described. Scheme 1 depicts the synthesis ofthe lead diketopiperazine targets 4 and 5. The chemistry employed isbased on Schölkopf chiral auxiliary chemistry and provides yields on thekilogram scale. Protection of the thiol (—SH group) moiety in thecysteine is required to insure the formation of the Schölkopf chiralauxiliary and prevent other cyclization reactions. Thiol protection isaccomplished by using either tert-butyl alcohol (in the presence ofhydrochloric acid), phenylsulfenyl chloride or triphenyl methyl chloride(trityl chloride). Thiol protected cysteine is converted via 2, usingthe ethyl ester (methyl ester may also be used) of the desired aminoacid, and undergoes intramolecular cyclization to produce the prodrug 3.Deprotection of the thiol group produces the lead diketopiperazinetarget 4.

Depicted in FIG. 1 are exemplary compounds that can be made usingnaturally occurring L-amino acids and D-isomers thereof. Alkylation ofcarboxyl groups on target 4 produces another prodrug 5 (Scheme 1).Furthermore, the dealkylation of the carboxyl group on 5 throughhydrolysis provides prodrugs 6a and 6b and eventually 4.

The synthesis of the symmetrical cystine prodrugs is preferablyaccomplished by carrying out the thiol deprotection step in either a)ethanol with a catalytic amount of mercaptoethanol for thephenylsulfenyl protected thiol or b) pyridine using a catalytic amountof iodine for the triphenyl methane protected thiol, as shown in Scheme2. However, the free thiol diketopiperazine can be used to producesymmetrical cystine prodrugs in ethanol and the presence of a catalyticamount of iodine. An exemplary cystine prodrug, the cysteine/glycinedimer, is further depicted in Scheme 2.

The synthesis of hetero (unsymmetrical) disulfide dimers is preferablyaccomplished by using a one-pot reaction with 1-chlorobenzotriazole, asshown in Scheme 3. An alternate method involves using a catalytic amountof iodine in the presence of an equal molar amount of any two triphenylmethane protected thiol cysteine prodrugs. The desired target can beseparated and purified using simple column chromatography.

Unsymmetrical disulfides can be synthesized from any two sulfide ligandsprovided by the above-described chemistries. Accordingly, the inventionencompasses symmetrical and unsymmetrical disulfide dimers synthesizedfrom the combination of any two sulfide monomers described herein.

The present method of synthesizing prodrugs according to the inventionhas many advantages over previous routes including, but not limited to:a) same synthetic route leads to both monomers and dimers (cysteine andcystine prodrugs); b) protection of the thiol group prevents side(cyclization) reactions; c) the initial monomer synthesis eliminatesproblems associated with multiple functional groups; d) the occurrenceof undesired intramolecular and intermolecular side reactions isdecreased; e) and the described route can be easily expanded toincorporate additional amino acids.

Particularly preferred cysteine monomers (prodrugs) according to theinvention are shown in FIG. 1, boldfaced and underlined. These compoundsare preferred either for advantages in partition coefficients (valine,proline), active transport (phenylalanine, proline), or breakdownproducts (cysteine, glycine). All targets synthesized from thediketopiperazine moiety are eventually cleaved and/or metabolized byeither intra- or extra-cellular mechanisms to produce cysteine orcystine, which can then be used in the cystine-glutamate antiporter.FIG. 2 depicts general chemical formulas for certain cysteine andcystine prodrugs encompassed by the present invention.

Accordingly, the invention provides a cysteine prodrug having thestructure:

wherein: R¹ and R² are independently selected from OH, ═O, or a branchedor straight chain C₁ to C₅ alkoxyl group, with the caveat that when ═Ois selected the nitrogen atom adjacent the carbonyl group thusly formedbears a H and a single bond joins the adjacent nitrogen to said carbonylgroup; R³ is H, a branched or straight chain C₁ to C₅ alkyl, anitrobenzenesulfonyl, a trityl, an aryl thio, an aryl, an alkylthio, anacyl, a benzoyl, a thio acyl, a thio benzoyl, or a benzyl group; and R⁴is selected from the side chain groups of the natural L-amino acids cys,gly, phe, pro, val, ser, arg, asp, asn, glu, gln, ala, his, ile, leu,lys, met, thr, trp, tyr, or D-isomers thereof, with the caveat that whenR⁴ is the side chain group of the natural L-amino acid gly, R¹ and R²are not both selected to be ═O; or a cystine dimer of said prodrughaving the structure:

wherein: R¹, R^(2,) R⁵ and R⁶ are independently selected from OH, ═O, ora branched or straight chain C₁ to C₅ alkoxyl group, with the caveatthat when ═O is selected the nitrogen atom adjacent the carbonyl groupthusly formed bears a H and a single bond joins the adjacent nitrogen tosaid carbonyl group; and R⁴ and R⁷ are independently selected from theside chain groups of the natural L-amino acids cys, gly, phe, pro, val,ser, arg, asp, asn, glu, gln, ala, his, ile, leu, lys, met, thr, trp,tyr, or D-isomers thereof, with the caveat that when R⁴ and R⁷ are boththe side chain group of the natural L-amino acid gly, R¹, R^(2,) R⁵ andR⁶ shall not all be selected to be ═O.

In certain preferred embodiments, the cysteine prodrug according to theinvention has the structure:

The cysteine prodrug may alternatively be provided in the form of acystine dimer. Certain preferred cystine dimers according to theinvention have the structure: the form of the cystine dimer having thestructure:

The invention provides synthetic routes for the synthesis of cystinedimers having identical R⁴ and R⁷ groups or, alternatively, mixed ornon-identical R⁴ and R⁷ groups. Cystine dimers of the invention maytherefore be of either symmetric or asymmetric design.

In certain cysteine prodrugs or cystine dimers of the invention, atleast one R⁴ or R⁷ group is the side chain of cysteine and the reactivemoiety thereof is further protected by a branched or straight chain C₁to C₅ alkyl, a nitrobenzenesulfonyl, a trityl, an aryl thio, an aryl, analkylthio, an acyl, a benzoyl, a thio acyl, a thio benzoyl, or a benzylgroup.

Upon administration to a subject, compounds according to the inventionpass largely intact through first pass metabolism and then arehydrolyzed (cleaved) into the corresponding amino acids by peptidases incells contained within the CNS. Accordingly, prodrugs are chemicalentities that are readily convertible in vivo to become pharmaceuticallyactive.

Scheme 4 and Scheme 5 illustrate yet another approach provided by theinvention in which L-cysteine is protected as acyl analogs with alkylesters to improve the partition coefficient (Log P) and circulatory halflife in the blood to provide improved passive delivery into the brainthrough the blood brain barrier.

In Scheme 5, glycine is incorporated into some of the protected cysteineanalogs (17) to provide a more efficient method of delivery of bothamino acids. Various alkyl alcohols are incorporated into targets fromScheme 4 (12). Symmetrical cystine targets are synthesized from thecorresponding cysteine analogs by the addition of a catalytic amount ofiodine. Again, all prodrugs are hydrolyzed (cleaved) into the activecorresponding amino acid in vivo. The molecules described in Scheme 4and Scheme 5 result in more exposure and increased brain levels ascompared to previous versions. It is noteworthy that this approachalters the partition coefficient by completely protecting thecysteine/cystine moiety. Synthetic challenges, such as solubility andstability of resulting intermediates and targets, have previouslyprevented others in the field from obtaining protected products insignificant quantities, even for research studies. FIG. 3 depictsgeneral chemical formulas for certain protected cysteine analogsencompassed by the present invention.

Accordingly, the invention further encompasses protected cysteineanalogs having the structure:

or a cystine dimer of the protected cysteine analog having thestructure:

wherein R¹ through R⁶ are independently selected from a branched orstraight chain C₁ to C₅ alkyl, a phenyl, or a benzyl group.

Preferable protected cysteine analogs according to the invention havethe structure:

Alternatively, protected cysteine analogs may be provided in the form ofthe corresponding cystine dimers. Certain preferred cystine dimers havethe structures:

Relative to the protected cysteine analogs, the invention furtherprovides a method of reducing schizophrenia in a subject byadministering to a subject an effective amount of a protected cysteineanalog or cystine dimer thereof according to the invention, wherebyschizophrenia is reduced in said subject. Administration is preferablyvia the oral route.

Of course, the invention further encompasses pharmaceutical compositionscontaining a protected analog or dimer thereof in combination with apharmaceutically-acceptable carrier. Methods offormulating/manufacturing such pharmaceutical compositions for thetreatment of schizophrenia or for reducing drug craving in a subject arealso within the invention's scope.

In certain preferred embodiments, the compounds of the invention will beprovided as pharmaceutically acceptable salts. Other salts may, however,be useful in the preparation of the compounds according to the inventionor of their pharmaceutically acceptable salts. Suitable pharmaceuticallyacceptable salts of the compounds of this invention include acidaddition salts which may, for example, be formed by mixing a solution ofthe compound according to the invention with a solution of apharmaceutically acceptable acid such as hydrochloric acid, sulphuricacid, methanesulphonic acid, fumaric acid, maleic acid, succinic acid,acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid,carbonic acid or phosphoric acid. Furthermore, where the compounds ofthe invention carry an acidic moiety, suitable pharmaceuticallyacceptable salts thereof may include alkali metal salts, e.g. sodium orpotassium salts, alkaline earth metal salts, e.g. calcium or magnesiumsalts; and salts formed with suitable organic ligands, e.g. quaternaryammonium salts.

Where the compounds according to the invention have at least oneasymmetric center, they may accordingly exist as enantiomers. Where thecompounds according to the invention possess two or more asymmetriccenters, they may additionally exist as diastereoisomers. It is to beunderstood that all such isomers and mixtures thereof in any proportionare encompassed within the scope of the present invention.

The invention also provides pharmaceutical compositions comprising oneor more compounds of this invention in association with apharmaceutically acceptable carrier. Preferably these compositions arein unit dosage forms such as tablets, pills, capsules, powders,granules, sterile parenteral solutions or suspensions, metered aerosolor liquid sprays, drops, ampoules, auto-injector devices orsuppositories; for oral, parenteral, intranasal, sublingual or rectaladministration, or for administration by inhalation or insufflation. Itis also envisioned that the compounds of the present invention may beincorporated into transdermal patches designed to deliver theappropriate amount of the drug in a continuous fashion.

For preparing solid compositions such as tablets, the principal activeingredient is mixed with a pharmaceutically acceptable carrier, e.g.conventional tableting ingredients such as corn starch, lactose,sucrose, sorbitol, talc, stearic acid, magnesium stearate, dicalciumphosphate or gums, and other pharmaceutical diluents, e.g. water, toform a solid preformulation composition containing a homogeneous mixturefor a compound of the present invention, or a pharmaceuticallyacceptable salt thereof. When referring to these preformulationcompositions as homogeneous, it is meant that the active ingredient isdispersed evenly throughout the composition so that the composition maybe easily subdivided into equally effective unit dosage forms such astablets, pills and capsules. This solid pre-formulation composition isthen subdivided into unit dosage forms of the type described abovecontaining from 0.1 to about 500 mg of the active ingredient of thepresent invention. Typical unit dosage forms contain from 1 to 100 mg,for example, 1, 2, 5, 10, 25, 50 or 100 mg, of the active ingredient.The tablets or pills of the novel composition can be coated or otherwisecompounded to provide a dosage affording the advantage of prolongedaction. For example, the tablet or pill can comprise an inner dosage andan outer dosage component, the latter being in the form of an envelopeover the former. The two components can be separated by an enteric layerwhich, serves to resist disintegration in the stomach and permits theinner component to pass intact into the duodenum or to be delayed inrelease. A variety of materials can be used for such enteric layers orcoatings, such materials including a number of polymeric acids andmixtures of polymeric acids with such materials as shellac, cetylalcohol and cellulose acetate.

The liquid forms in which the novel compositions of the presentinvention may be incorporated for administration orally or by injectioninclude aqueous solutions, suitably flavored syrups, aqueous or oilsuspensions, and flavored emulsions with edible oils such as cottonseedoil, sesame oil, coconut oil or peanut oil, as well as elixirs andsimilar pharmaceutical vehicles. Suitable dispersing or suspendingagents for aqueous suspensions include synthetic and natural gums suchas tragacanth, acacia, alginate, dextran, sodium caboxymethylcellulose,methylcellulose, polyvinylpyrrolidone or gelatin.

The compounds according to the present invention exhibit schizophreniareducing/alleviating activity, as demonstrated by standard protocols.For example, efficacy of the present inventive compounds in theschizophrenia context has been demonstrated by assaying startle responseto a load stimulus (pulse) when preceded by a pre-pulse stimulus.Accordingly, another aspect of the invention provides a method for thereduction of schizophrenia in a subject in need of such treatment byadministration of an effective amount of compound according to theinvention or a precursor thereof. In the treatment of schizophrenia,suitable dosage level (i.e, an effective amount) is about (1-5000)mg/kg, per day, preferably about (30-3000) mg/kg per day, and especiallyabout (50-1000) mg/kg per day. The compounds may be administered on aregimen of 1 to 4 times per day, or on a continuous basis.

Accordingly, the present invention further provides a method of reducingschizophrenia in a subject. Such a method includes steps ofadministering to the subject an effective amount of a cysteine prodrugor cystine dimer thereof according to the invention, wherebyschizophrenia is reduced in the subject. Administration is preferablyaccomplished by oral delivery.

As well, the compounds according to the present invention may alsoexhibit the ability to reduce drug cravings. This desirable activity canbe shown in animal models involving drug-seeking behavior produced bystress, drug-paired cues, or a cocaine priming injection. Accordingly,yet another aspect of the invention is directed to a method of reducinga drug craving in a subject in need thereof. Such a method includes thestep of administering an effective amount of a compound having thechemical structure of compound according to the invention, or aprecursor thereof, to the subject whereby the drug craving is reduced inthe subject. In the treatment of drug cravings, suitable dosage level(i.e., effective amount) is about (1-5000) mg/kg, per day, preferablyabout (30-3000) mg/kg per day, and especially about (50-1000) mg/kg perday.

The invention therefore provides a method of reducing drug craving in asubject. Such a method includes steps of administering to the subject aneffective amount of a cysteine prodrug or cystine dimer of theinvention, whereby drug craving is reduced in the subject. Again,administration is preferably via the oral route.

The following examples are, of course, offered for illustrative purposesonly, and are not intended to limit the scope of the present inventionin any way. Indeed, various modifications of the invention in additionto those shown and described herein will become apparent to thoseskilled in the art from the foregoing description and the followingexamples and fall within the scope of the appended claims.

In the following examples, the compounds where named based on thefollowing criteria: Cysteine prodrugs (monomers) were assigned names as(Assigned number from Scheme—Amino acid incorporated; a=glycine,b=phenylalanine, c=proline, d=valine, e=cysteine) (i.e. 3c-a: Target 3cfrom Scheme 1 with glycine incorporated) or alternatively (Assignednumber from Scheme with a “letter” indicating the amino acidincorporated) (i.e. 4a: Target 4 from Scheme 1 with glycineincorporated), Cystine prodrugs were named as (Assigned number fromScheme with a “letter” indicating the amino acid incorporated) (i.e. 7b:Target 7 from Scheme 2 with phenylalanine incorporated) or alternativelyas (Assigned number from Scheme with a “letter” indicating the aminoacid incorporated—dimer) (i.e. 5a-dimer: The dimer of Target 5 fromScheme 1 with glycine incorporated). Unsymmetrical Cystine prodrugs werenamed as (Assigned number from Scheme—Amino acid incorporated (monomer1)—Amino acid incorporated (monomer 2)) (i.e. 11-a-b: Target 11 fromScheme 3 with glycine incorporated into monomer 1 and phenylalanineincorporated into monomer 2).

EXAMPLES Example 1 Experimental for Scheme 1 Compounds

Preparation of p-Tolyl hypochlorothioite

Under a nitrogen atmosphere, N-chloro-succinimide (48.1 g, 0.36 mole)was slurried in 200 ml of methylene chloride. While stirring at roomtemperature, 4-methylbenzenethiol (29.8 g, 0.24 mole) was added; (2 ginitial addition to start reflux and the remainder at a rate to maintainreflux approximate 10 min.) The clear solution which resulted was thenstirred at room temperature for 30 minutes. A small amount ofprecipitate which formed was removed by filtration. The filtrate,assumed to contain the theoretical quantity of 4-methylbenzenesulfenylchloride (38.1 g, 0.24 mole), was used immediately and directly in thenext step. Alternatively, 4-methylbenzene-sulfenyl chloride was isolatedby evaporation to an solid to its further use.

(R)-2-amino-3-(phenyldisulfanyl)propanoic acid (1b)

To a solution of L-cysteine hydrochloride mono-hydrate (47 g, 0.3 mol)in absolute ethanol (900 mL) was added powdered sodium bicarbonate (30g, 0.36 mol) at 0° C. in one portion. Phenylsulfenyl chloride (50 g,0.345 mol) was added dropwise with stirring to the mixture. After thecomplete addition of the reagent, the reaction mixture was allowed tostand at room temperature and the sodium chloride which was producedduring the reaction was removed by filtration. After basifying themixture by the addition of pyridine (38 mL) into the filtrate, the fineprecipitate which formed was allowed to stand for a couple of hours,then filtrated and washed well with ethanol and dried to provide thecrude product as a white solid. After recrystallization from aqueous HCl(0.5 N, 4000 mL), the final product S-thiol-phenyl-L-cysteine (1b) wasobtained (52 g) in 76% yield as colorless plates. 1b: m.p. 192° C.(decomp). ¹H NMR (300 MHz, CD₃CO₂D): δ 3.53-3.76 (m, 2H), 4.89 (t, 1H),7.26-7.88 (m, 5H); ¹³C NMR (75.5 MHz, CD₃CO₂D): δ 35.5, 52.5, 127.6,128.5, 129.1, 129.3, 133.5, 171.6. This material was employed directlyin the next step.

2-Amino-3-tritylsulfanyl-propionic acid (S-Trityl-L-cysteine) (1c)

L-Cysteine hydro-chloride (100 g, 0.634 mol) and trityl chloride (270 g,0.969 mol) were stirred in DMF (400 mL) for 2 days at room temperature.A 10% sodium acetate solution (3.5 L) was then added dropwise and thewhite precipitate which formed was filtered and washed with distilledwater. Afterward, the residue was stirred in acetone at 50° C. for 30min after which it was cooled to 0° C. and filtered. The precipitate waswashed with a little acetone and diethyl ether and dried in vacuo.S-Trityl-L-cysteine 1c (205 g, 89%) was obtained as a white powder. 1c:m.p. 192° C. (decomp); ¹H NMR (300 MHz, DMSO-d₆) δ 2.45 (dd, 1H, J=9 Hz,12 Hz), 2.58 (dd, 1H, J=4.4 Hz, 12 Hz), 2.91 (m, 1H), 7.22-7.36 (m,15H); ¹³C NMR (75.5 MHz, DMSO-d₆): δ 33.8, 53.7, 66.4, 127.1, 127.8,128.1, 128.4, 129.5, 144.5, 168.4. This material was directly used inthe next step without further purification.

(R)-4-((phenyldisulfanyl)methyl)oxazolidine-2,5-dione (2b)

To a rapidly stirred (over-head stirrer) suspension ofS-thiol-phenyl-L-cysteine (1b) (57.5 g, 0.25 mol) in THF (250 mL) wasadded solid triphosgene (26 g, 88 mmol) in one portion at 45-50° C.(before addition, remove the heating mantle). When the temperature dropsto 45° C., put the heating mantle back on and maintain the insidetemperature around 45-50° C. until the solution becomes homogeneous.After the removal of the heating mantle, the solution was purged withargon overnight into a NaOH bubbler to remove any residual phosgene. Thesolvent was evaporated in vacuo and this provided anhydride 2b (55 g) in85% yield. 2b: m.p. 217° C. (decomp); ¹H NMR (300 MHz, CDCl₃) δ2.90-2.98 (m, 1H), 3.30 (d, 1H, J=12 Hz), 4.68 (d, 1H, J=9 Hz), 6.01 (s,1H), 7.34-7.58 (m, 5H); ¹³C NMR (75.5 MHz, CD₃Cl₃): δ 39.4, 56.5, 128.3,128.9, 129.5, 135.2, 150.8, 167.7. Due to the unstable nature of thisanhydride, it was stored in the refrigerator overnight under anatmosphere of argon and used immediately the next day without furtherpurification.

4-Tritylsulfanylmethyl-oxazolidine-2,5-dione (2c)

was prepared following the procedure for preparation of 2b as a brownoil in 85% yield. 2c: ¹H NMR (300 MHz, CDCl₃) δ 2.70-2.85 (m, 2H),3.47-3.56 (m, 1H), 5.62 (s, 1H), 7.07-7.73 (m, 15H). This material wasdirectly used in the next step without further purification.

Representative Procedure for Synthesis of Diketopiperazine Targets

2,5-piperazinedione, 3-(mercaptomethyl)-(4a)

a). A solution of the N-carboxy-anhydride 2b (35.7 g, 0.14 mol) in THF(160 mL) was added dropwise to a vigorously stirred (overhead stirrer)mixture of glycine ethyl ester hydrochloride (28 g, 0.16 mol), freshlydistilled triethylamine (20.4 g, ˜28 mL, 0.20 mol) and dry chloroform(240 mL) at −78° C. in a three-neck flask (2 L). The reaction mixturewas allowed to warm to 0° C. over 8 h, and then was stirred at rt for 12h, after which the reaction solution was filtered to remove thetriethylamine hydrochloride which precipitated. The filtrate was thenconcentrated under reduced pressure (<40° C.) and the crude dipeptideester was used for the preparation of the diketopiperazine 4a withoutfurther purification. ¹H NMR (300 MHz, CDCl₃): δ 1.29 (t, 3H), 1.93 (br,2H), 2.74-2.82 (m, 1H), 3.40 (dd, 1H), 3.73 (dd, 1H), 4.03-4.19 (m, 2H),4.19-4.26 (m, 2H), 7.34-7.58 (m, 5H). b). The crude dipeptide ester(37.6 g, 0.12 mol) was heated in refluxing toluene (1000 mL) for 12 hand then cooled down to rt and kept at 0° C. for 16 h. The bislactam 4awhich precipitated was isolated by vacuum filtration, washed with ether(3×150 mL), and dried under vacuum at 100° C. to provide purediketopiperazine 4a (10.0 g) in 45% yield. The resulting filtrateproduced from washing the desired diketopiperazine was evaporated undervacuum and toluene (800 mL) was added to the residue. The toluenesolution was heated at reflux for another 40 h (under argon) and thenthe above steps were repeated to collect another 5-8 grams ofdiketopiperazine 4a (combined yield, 73%). 4a: m.p. 258° C.; ¹H NMR (300MHz, DMSO-d₆): δ 3.09-3.26 (m, 2H), 3.68-3.88 (m, 2H), 4.10 (s, 1H),8.17 (s, 1H), 8.19 (s, 1H); ¹³C NMR (500 MHz, DMSO-d₆): δ 43.5, 44.7,54.3, 166.2, 166.6; EIMS (m/e, relative intensity) 160(M⁺, 12), 140(5),126(72), 114(100), 97(20), 85(30).

3-Phenyldisulfanylmethyl-piperazine-2,5-dione (3b-a)

c). The solution which resulted from step b above was cooled to 0° C.and keep at 0° C. for 12 h. The precipitate which resulted was filteredand provided phenyl-thiol analog 3b-a in 30% yield. 3b-a: ¹H NMR (300MHz, DMSO-d₆): δ 3.09-3.21 (m, 2H), 3.65-3.82 (m, 2H), 4.10 (s, 1H),7.11-7.55 (m, 5H), 8.18 (s, 1H), 8.20 (s, 1H); ¹³C NMR (75.5 MHz,DMSO-d₆): δ 43.5, 47.8, 54.2, 125.6, 127.7, 128.2, 129.5, 166.2, 166.6;EIMS (m/e, relative intensity) 268 (M⁺, 55), 250(35), 218(68), 159(66),141(80), 126(70).

(3R,6R)-3-benzyl-6-(mercaptomethyl)piperazine-2,5-dione (4b)

was prepared in 75% yield following the procedure for preparation of 4aand obtained as a light yellow solid. 4b: m.p. >265 (decomp.); ¹H NMR(300 MHz, DMSO-d₆) δ 1.26 (d, J=6.99 Hz, 1H), 3.05-3.49 (m, 2H),3.66-3.89 (m, 3H), 4.10 (s, 1H), 7.13-7.31 (m, 5H), 8.23 (s, 1H), 8.28(s, 1H); ¹³C NMR (75.5 MHz, CDCl₃) δ 19.0, 37.9, 44.7, 48.1, 51.2, 54.4,126.5, 129.1, 129.4, 165.9, 166.5. EIMS (m/e, relative intensity) 250(M⁺, 10), 216(12), 160(5), 113(11), 91(100).

(6R)-3-isopropyl-6-(mercaptomethyl)piperazine-2,5-dione (4d)

was prepared in 74% yield following the procedure for preparation of 4aand obtained as a white solid. 4d: m.p. >275° C.; ¹H NMR (300 MHz,DMSO-d₆) δ 0.84 (dd, J=7.14, 6.63 Hz, 3H), 0.94 (dd, J=8.07, 6.9 Hz,3H), 2.17-2.20 (m, 1H), 3.07-3.18 (m, 2H), 3.73 (s, 1H), 4.22 (s, 1H),8.12 (s, 1H), 8.18s (s, 1H); ¹³C NMR (75.5 MHz, CDCl₃) δ 17.5, 18.8,42.9, 53.9, 59.7, 166.7, 167.2; FIRMS m/z C₁₀H₁₈N₂O₂S₂(M−F)⁺ calcd201.0698, found 201.0691.

(6R)-3-(tert-butylthiomethyl)-6-(mercaptomethyl)piperazine-2,5-dione(4e)

was prepared in 70% yield following the procedure for preparation of 4aand obtained as a yellow solid. 4e: m.p. >280° C. (decomp.); ¹H NMR (300MHz, DMSO-d₆) δ 1.25 (s, 9H), 2.88-2.92 (m, 1H), 3.03-3.10 (q, J=7.5 Hz,1H), 3.18-3.21 (m, 1H), 3.51 (d, J=14.4 Hz, 1H), 4.14 (s, 2H), 8.13 (s,1H), 8.24 (s, 1H); ¹³C NMR (75.5 MHz, CDCl₃) δ 31.1, 32.1, 43.2, 47.8,54.1, 54.9, 166.3, 170.8; EIMS (m/e, relative intensity) 262 (M⁺, 30),228(40), 206(45), 173(50), 160(70), 126(100); HRMS m/z C₁₀H₁₈N₂O₂S₂(M+H)⁺ calcd 263.0482, found 263.0489.

(3R,8aR)-3-((phenyldisulfanyl)methyl)hexahydropyrrolo[1,2-a]pyrazine-1,4-dione(3b-c)

was prepared in 82% yield following the procedure for preparation of3b-a and obtained as a yellow solid. 3b-c: m.p. 120° C.; ¹H NMR (300MHz, CDCl₃) δ 1.66-2.02 (m, 1H), 2.03-2.11 (m, 2H), 2.36 (m, 1H),2.80-2.89 (m, 1H), 3.54-3.62 (m, 3H), 4.07-4.10 (m, 1H), 4.39 (dd,J=1.83, 1.77 Hz, 1H), 6.35 (s, 1H), 7.28-7.57 (m, 5H); ¹³C NMR (75.5MHz, CDCl₃) δ 22.5, 28.2, 38.5, 45.4, 53.3, 59.1, 127.8, 128.6, 129.2,135.6, 164.3, 169.0.

3-Tritylsulfanylmethyl-piperazine-2,5-dione (3c-a)

was prepared following the similar procedure for preparation of 4a.3c-a: m.p. 225-227° C. [α]_(D) ²⁶=+7.8° (c=1.05, CHCl₃). ¹H NMR (300MHz, CDCl₃) δ 2.73-2.91 (m, 2H), 3.12 (d, 1H, J=12.3 Hz), 3.95 (s, 1H),5.80 (s, 1H), 5.82 (s, 1H), 7.20-7.62 (m, 15H). ¹³C NMR (75.5 MHz,CDCl₃): δ 35.9, 44.8, 53.0, 126.9, 128.1, 129.4, 144.0, 166.6. Thismaterial was directly used in the next step without furtherpurification.

Representative Procedure for Synthesis of Dialkylated Diketopiperazine

(3,6-Diethoxy-2,5-dihydro-,pyrazin-2-yl)-methanethiol (5a) Preparationof Triethyloxonium tetrafluoroborate

(Note: Triethyloxonium tetra-fluoroborate is an expensive reagent;however, it is relatively easy to prepare even on large scale). Athree-neck flask (500 mL), pressure equilibrating dropping funnel (125mL) and a condenser were dried in an oven at 150° C. and assembled whilehot under an atmosphere of argon. When the equipment had cooled to rt,ether [(100 mL) which had been previously dried over sodium benzophenoneketyl] and boron trifluoride diethyletherate (91 g, ˜87 mL, 64 mmol)were combined [Note: On this scale the colorless BF₃ etherate wasobtained from a freshly opened new bottle. If the reagent was slightlyyellow or if the reaction was scaled down, the BF₃ etherate needed to bevacuum distilled first]. The ethereal solution which resulted was heatedto a gentle reflux after which dry epichlorohydrin (48.8 g, ˜41 mL, 51.8mmol) was added dropwise over 1 h. The mixture was heated at reflux foran additional 1 h and allowed to stand at rt (under argon) overnight.The ether was removed by applying a positive pressure of argon in oneneck of the flask while forcing the ether out through a filter stick(fitted glass tube) inserted into another neck of the flask and into acollection flask. The slightly yellow solid which remained in the flaskwas rinsed twice in the same manner with anhydrous ether (3×50 mL) toprovide a crystalline white solid. The solid was not weighed butdirectly used in the next step. The following sequence was based on theyield of this reaction process at the level of 80-85%.

Dry CH₂Cl₂ (100 mL) was added to the flask (500 mL) which contained thefreshly prepared triethyloxonium tetrafluoroborate (˜42 g, 336 mmol)from the previous reaction (under argon). To this solution was added thediketopiperazine 4a (5 g, 31.2 mmol) in portions with stirring (overheadstirrer). After 2 h the reaction mixture became homogenous. The solutionwas stirred at rt under argon for 72 h after which the mixture was addedvia a cannula to an aq solution of NH₄OH (14%, 100 mL) mixed with ice(100 g). The organic layer was washed with a saturated aq solution ofNaHCO₃(2×50 mL) and brine (80 mL) after which it was dried (K₂CO₃).After filtration the solvent was removed under reduced pressure toprovide the bis-ethoxy lactim ether 5a as a clear yellow liquid that wasfurther purified by flash chromatography (EtOAc:Hexane=1:4) in 71% yield(4.8 g, 22 mmol). 5a: [α]_(D) ²⁶=+52.2° (c=2.5, CHCl₃). ¹H NMR (300 MHz,CDCl₃) δ 1.32-1.36 (m, 6H), 3.27-3.30 (m, 3H), 4.08-4.22 (m, 6H), 4.39(s, 1H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.7, 46.3, 47.5, 56.1, 61.5,61.6, 162.7, 163.6; HRMS m/z C₉H₁₆N₂O₂S (M+H)⁺ calcd. 217.2982, found217.2990.

(3R,6R)-6-Benzyl-5-ethoxy-3-(ethylthiomethyl)-1,6-dihydropyrazin-2(3H)-one(6b-b)

was prepared in 30% yield following the procedure for preparation of 5busing only 1 equiv. of triethyloxonium tetrafluoroborate and obtained asa yellow solid. 6b-b: m.p. 118° C.; ¹H NMR (300 MHz, CDCl₃) δ 1.19 (t,J=7.41 Hz, 3H), 1.30 (t, J=7.08 Hz, 3H), 2.37-2.45 (m, 3H), 2.82-3.01(m, 1H), 2.95 (d, J=3.09 Hz, 1H), 3.03 (d, J=3.12 Hz, 1H), 3.23 (q,J=32.6, 5.1 Hz, 2H), 4.14-4.19 (m, 2H), 4.46-4.47 (m, 1H), 6.19 (s, 1H),7.17-7.29 (m, 5H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.1, 14.5, 25.7, 35.4,39.8, 50.6, 60.0, 61.6, 126.5, 127.8, 130.2, 136.7, 157.8, 170.1; FIRMSm/z (M+H)⁺ calcd. 305.1515, found 305.1522.

Example 2 Representative Procedure for Synthesis ofBis-Dipiperazinedione

Bis[2,5-Piperazinedione, 3-(mercaptomethyl)-] (7a)

The trityl protected diketo-piperazine 3c-a (1.5 g, 3.73 mmol) wasdissolved in a solution of methylene chloride (20 mL) and methanol (40mL) with stirring. Pyridine (1.2 mL, 15 mmol) was then added to theresulting mixture, followed by a solution of iodine (0.97 g, 3.8 mmol)in methanol (5 mL). The mixture was allowed to stir for 1 h at roomtemperature. No precipitate had formed by this time; however, TLCanalysis indicated that the reaction was proceeding slowly by theappearance of a new spot under the starting material (UV light). Aprecipitate began to form within 2 h after concentrating the solution toa volume of 10 mL and methanol (30 mL) was added to result in a totalvolume of 40 mL. The solution was stirred an additional 23 h and theprecipitate was filtered off. The solid was washed with cold methanoland then decolorized by shaking with 10% aqueous sodium bisulfite (10mL). The precipitate was filtered and dried to yield dimer 7a as whitesolid (680 mg, 57%). 7a: m.p. >300° C. ¹H NMR (300 MHz, DMSO-d₆) δ3.11-3.21 (m, 2H), 3.70 (d, 1H, J=0.96 Hz), 3.73 (d, 1H, J=0.99 Hz),4.11 (s, 1H), 8.17 (s, 1H), 8.19 (s, 1H); ¹³C NMR (75.5 MHz, DMSO-d₆): δ44.0, 45.2, 54.8, 166.7, 167.1; HRMS m/z (M+H)⁺ calcd. 319.0535, found319.0533.

(3S,6S)-3-Benzyl-6-(((((2R,5R)-5-benzyl-3,6-dioxopiperazin-2-yl)methyl)disulfanyl)methyl)piperazine-2,5-dione(7b)

was prepared in 63% yield following the procedure for preparation of 7aand obtained as a yellow solid. 7b: m.p. >280° C. (decomp.); ¹H NMR (300MHz, CDCl₃) δ 1.29 (s, 9H), 2.85-2.92 (m, 2H), 3.10-3.13 (m, 2H), 4.14(s, 2H), 8.12 (s, 2H); ¹³C NMR (75.5 MHz, CDCl₃): δ 31.1, 32.1, 42.5,43.2, 53.9, 54.1, 166.2, 166.3.

(3R,3′R,6R,6′R)-6,6′-disulfanediylbis(methylene)bis(3-isopropylpiperazine-2,5-dione)(7d)

was prepared in 65% yield following the procedure for preparation of 7aand obtained as a white solid. 7d: m.p. 270° C.; ¹H NMR (300 MHz, CDCl₃)δ 0.86 (d, J=6.75 Hz, 3H), 0.96 (d, J=7.05 Hz, 3H), 2.17-2.21 (m, 1H),3.07-3.19 (m, 2H), 3.72 (s, 1 h), 4.33 (s, 1H), 8.11 (s, 1H), 8.17 (s,1H); ¹³C NMR (75.5 MHz, CDCl₃): δ 17.5, 18.8, 31.4, 42.9, 53.9, 59.7,166.7, 167.2; HRMS m/z (M+H)⁺ calcd. 403.1474, found 403.1479.

(3R,6S)-3-(tert-Butylthiomethyl)-6-(((((2R,5S)-5-(tert-butylthiomethyl)-3,6-dioxo-piperazin-2-yl)methyl)disulfanyl)methyl)piperazine-2,5-dione(7e)

was prepared in 65% yield following the procedure for preparation of 7aand obtained as a yellow solid. 7e: m.p. 278° C.; ¹H NMR (300 MHz,CDCl₃) δ 1.29 (s, 9H), 2.85-2.92 (m, 2H), 3.10-3.13 (m, 2H), 4.14 (s,2H), 8.12 (s, 2H); ¹³C NMR (75.5 MHz, CDCl₃): δ 31.1, 32.1, 42.5, 43.2,53.9, 54.1, 166.2, 166.3.

Representative Procedure for Synthesis ofBis[(3,6-Diethoxy-2,5-dihy-dro-pyrazin-2-yl)-methanethiol] (5a-dimer)

To the bis-ethoxy lactim ether 5a (400 mg, 1.85 mmol) in dry EtOH (10mL) was added a catalytic amount of I₂ (50 mg, 10% mmol) at rt. Themixture was stirred for 6˜12 h under air until the analysis (TLC, silicagel) indicated the reaction was complete (new spot appeared under S.M.on the TLC plate). The organic solvent was evaporated under reducedpressure. The mixture which resulted was dissolved into EtOAc (20 mL),washed with sat. sodium thiosulfate (5˜10 mL) and dried (Na₂SO₄). Thesolvent was then removed under reduced pressure which provided the dimer5a-dimer: ¹H NMR (300 MHz, CDCl₃) δ 1.32-1.36 (m, 6H), 3.27-3.30 (m,3H), 4.08-4.22 (m, 6H), 4.39 (s, 1H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.7,46.3, 47.5, 56.1, 61.5, 61.6, 162.7, 163.6; The NMR spectra wasidentical to its monomer except the S—H bond had disappeared. HRMS m/z(M+H)⁺ calcd. 431.1787, found 431.1790.

1,2-Bis(((2R,5R)-5-benzyl-3,6-diethoxy-2,5-dihydropyrazin-2-yl)methyl)disulfane(5b-dimer)

was prepared in 65% yield following the procedure for preparation of5a-dimer and obtained as a yellow liquid. 5b-dimer: ¹H NMR (300 MHz,CDCl₃) δ 1.26-1.35 (m, 6H), 2.45-2.57 (m, 1H), 3.05-3.22 (m, 2H),3.50-3.82 (m, 1H), 4.07-4.18 (m, 5H), 4.32-4.38 (m, 1H), 7.06-7.28 (m,5H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.3, 39.6, 42.9, 43.0, 54.9, 57.1,60.7, 60.8, 126.2, 126.5, 127.8, 137.0, 162.2, 162.6; The NMR spectrawas identical to its monomer except the S—H bond had disappeared. HRMSm/z (M+H)⁺ calcd. 611.2681, found 611.2677.

1,2-Bis(((2R,5R)-3,6-diethoxy-5-isopropyl-2,5-dihydropyrazin-2-yl)methyl)disulfane(5c-dimer)

was prepared in 60% yield following the procedure for preparation of5a-dimer and obtained as a colorless liquid. 5c-dimer: ¹H NMR (300 MHz,CDCl₃) δ 0.76-0.78 (m, 3H), 1.06-1.09 (m, 3H), 1.25-1.31 (m, 6H),2.18-2.23 (m, 1H), 2.82-3.01 (m, 1H), 3.21-3.45 (m, 1H), 3.54-3.70 (m,2H), 4.07-4.33 (m, 4H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.2, 17.3, 31.1,31.7, 45.2, 55.3, 60.5, 60.7, 161.0, 163.1; HRMS m/z (M+H)⁺ calcd.515.2726, found 515.2731.

Example 3 Alternative Route for Synthesis of AsymmetricBis-Dipiperazinedione

Bis[2,5-Piperazinedione, 3-(mercaptomethyl)-] (11-a-b)

The trityl protected diketo-piperazine 3c-a (246 mg, 0.5 mmol) and 3c-b(201 mg, 0.5 mmol) were dissolved in a solution of methylene chloride (5mL) and methanol (10 mL) with stirring. Pyridine (0.3 mL, 3.75 mmol) wasthen added to the resulting mixture, followed by a solution of iodine(126 mg, 0.5 mmol) in methanol (3 mL). The mixture was allowed to stirfor 1 h at room temperature. No precipitate had formed by this time;however, TLC analysis indicated that the reaction was proceeding slowlyby the appearance of a new spot under the starting material (UV light).A precipitate began to form within 2 h after concentrating the solutionto a volume of 2 mL and methanol (5 mL) was added to result in a totalvolume of 10 mL. The solution was stirred an additional 23 h and theprecipitate was filtered off. The solid was washed with cold methanol.The precipitate was filtered and dried to yield dimer 11-a-b as yellowsolid (120 mg, 60%). 11-a-b: ¹H NMR (300 MHz, DMSO-d₆) δ 2.89-2.91 (m,2H), 3.09-3.21 (m, 3H), 3.33-3.87 (m, 4H), 4.11 (s, 1 H), 4.21 (s, 1H),7.13-7.36 (m, 5H), 8.07 (s, 1H), 8.32 (s, 2H), 8.58 (s, 1H); ¹³C NMR(75.5 MHz, DMSO-d₆) δ 42.3, 42.6, 43.1, 44.7, 53.3, 54.2, 54.3, 55.8,127.2, 128.2, 130.6, 136.4, 165.9, 166.1, 166.5.

Example 4 Experimental for Scheme 4 and 5 Compounds RepresentativeProcedure for Synthesis of Protected Analogs (16)

N,S-Dibenzoyl-L-cysteine Ethyl Ester (16)

To a solution of pure L-cysteine ethyl ester hydrochloride (7.5 g, 40mmol) in pyridine (30 mL) precooled at 0° C., benzyol chloride (10 mL)was added. After being kept for 1 h at room temperature, the mixture waspoured onto ice. The precipitate was collected by filtration and wasrecrystallized from methanol in 88% yield (12 g). 16: m.p. 81° C.; ¹HNMR (300 MHz, CDCl₃): δ 1.41 (t, J=6 Hz, 3H), 3.40-3.48 (m, 1H),3.68-3.75 (m, 1H), 4.15 (q, J=7.11, 7.17 Hz, 2H), 4.62-4.70 (m, 1H),7.48-7.57 (m, 5H), 7.66-7.69 (m, 1H), 7.84-7.93 (m, 4H), 9.02 (d, J=7.8Hz, 1H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.4, 29.9, 52.6, 61.4, 127.2,127.7, 128.7, 129.5, 132.0, 133.8, 134.5, 136.4, 166.8, 170.5, 191.0;HRMS m/z (M+H)⁺ calcd. 358.1113, found 358.1106.

Representative Procedure for Synthesis of Compound, 18 (Protected Analog17 Coupling with Glycine)

Preparation of Phenyl acetyl-5-trityl-L-cysteine

To a suspension of S-trityl-L-cysteine 1c (4.4 g, 12 mmol) in chloroform(92 mL) containing triethylamine (2.7 g, 26.4 mmol) cooled in ice, wasadded a solution of phenylacetyl chloride (1.8 g, 12 mmol) in chloroform(20 mL). The mixture was stirred at 0-5° C. for 15 min. and at roomtemperature for 24 hrs. Water was added (100 mL) and pH was adjusted to1.5 with 5 N aqueous HCl. The aqueous phase was removed and the organicphase was washed with saturated sodium chloride (100 mL), dried (Na₂SO₄)and concentrated to give a white crystalline solid (4.9 g) in 85% yield.Phenyl acetyl-5-trityl-L-cysteine: m.p. 60-62° C.; [α]_(D)=+21.8° (c 2,CH₃OH); ¹H NMR (300 MHz, CDCl₃): δ 2.60-2.71 (m, 2H), 3.5 (s, 1 H),4.15-4.23 (m, 1H), 5.92 (d, J=6.48 Hz, 1H), 7.21-7.33 (m, 20H);); ¹³CNMR (75.5 MHz, CDCl₃): δ 32.9, 43.1, 51.4, 67.8, 126.8, 127.2, 127.4,127.8, 127.9, 128.4, 128.9, 129.1, 129.4, 144.1, 171.5, 172.5.

N-Carbobenzoxy-5-trityl-L-cysteinylglycine ethyl ester (17)

To a solution of glycine ethyl ester hydrochloride (1.25 g, 9 mmol) inchloroform (50 mL) and triethylamine (1.25 mL) was added phenylacetyl-5-trityl-L-cysteine (4.8 g, 10 mmol) andN,N′-dicyclohexycarbodiimide (2.1 g, 10 mmol). After stirred at roomtemperature overnight followed by addition of a few drops of 50% aceticacid the insoluble precipitate of dicyclohexylurea (1.7 g) was removedby filtration; the filtrate was washed successively with dilutehydrochloric acid, potassium hydrogen carbonate and water, dried oversodium sulfate and evaporated to dryness. The residue was treated withethyl acetate. Some undissolved material (dicyclohexylurea, 0.5 g) wasfiltered off and the filtrate was concentrated in vacuo to a smallvolume. Crystalline 17 was separated out in 85% yield. 17: m.p. 152° C.;¹H NMR (300 MHz, CDCl₃): δ 1.23-1.32 (m, 3H), 2.57-2.62 (m, 2H), 3.53(s, 1H), 3.87-3.91 (m, 2H), 4.13 (d, J=6.18 Hz, 1H), 4.15-4.23 (m, 2H),5.91 (d, J=7.41 Hz, 1H), 6.55 (s, 1H), 7.21-7.45 (m, 20H); ¹³C NMR (75.5MHz, CDCl₃): δ 14.0, 33.0, 41.3, 43.3, 51.9, 61.4, 67.0, 126.8, 127.3,127.9, 128.9, 129.3, 129.5, 134.1, 144.3, 169.1, 169.9, 171.1.

Bis[(R)-ethyl 2-(3-mercapto-2-(2-phenylacetamido)propanamido)acetate](18)

was prepared in 72% yield following the procedure for preparation of 7aand obtained as a yellow solid. 18: m.p. 98° C.; ¹H NMR (300 MHz,CDCl₃); 61.27-1.31 (m, 1H), 2.79-2.87 (m, 1H), 3.00-3.07 (m, 1H), 3.64(s, 2H), 3.68-3.76 (m, 1H), 3.96-4.16 (m, 1H), 4.04-4.23 (m, 2H),5.52-5.58 (m, 1 H), 6.56 (d, J=9.15 Hz, 1H), 7.25-7.35 (m, 5H),8.40-8.44 (s, 1H); ¹³C NMR (75.5 MHz, CDCl₃): δ 14.1, 41.1, 43.1, 46.3,53.0, 61.2, 127.2, 128.6, 129.5, 134.2, 169.1, 170.5, 171.5.

Example 5 PCP Dose-Dependently Alters Prepulse Inhibition and Impact ofN-Acetyl Cysteine on Sensorimotor Gating Deficits Produced by PCP

Sensorimotor gating, a process compromised in schizophrenic patients, isoften measured using prepulse inhibition whereby a mild auditorystimulus (prepulse, 2-15 db above background) precedes (100 ms) astartle-eliciting auditory stimulus (50 dB above background). Intactsensorimotor gating will result in suppression of the startle reflexwhen preceded by the prepulse. Since improvement in prepulse inhibitiontracks improvement in symptoms that are largely insensitive to currenttreatments, this paradigm has become one of the most commonly usedscreening paradigms. FIG. 4 illustrates the capacity of PCP to disruptprepulse inhibition, rendering the prepulse ineffective in suppressingthe startle reflex. PCP is commonly used to disrupt prepulse inhibitionbecause this abnormality, in addition to negative and cognitivesymptoms, are insensitive to generation antipsychotics thereby providingpredictive validity.

FIG. 5 illustrates the impact of N-acetyl cysteine on sensorimotorgating deficits produced by phencyclidine administered orally (left) ordirectly into the prefrontal cortex (right), which is likely thetherapeutic site of action for cysteine prodrugs. N=6-46/group. *indicate a significant difference from rats receiving PCP only (e.g., 0N-acetyl cysteine), Fisher LSD, p. 05.

Example 6 Efficacy of Compounds from Scheme 1 Relative to N-acetylcysteine in Reversing Pcp-Induced Deficits in Sensorimotor Gating inRats

FIG. 6 is a bar graph illustrating inhibition of a startle response inresponse to a load stimulus (pulse) when preceded by a pre-pulsestimulus (2-15 db above background). Prepulse inhibition is a commonlyused paradigm to screen antipsychotic agents for use in treatingschizophrenia. The pre-pulse stimulus presented at 15 dB abovebackground reduced the startle response in saline controls (S; N=46)by >60% relative to the response elicited following exposure to thepulse only. Rats pretreated with phencyclidine only (P; 1.25 mg/kg, SC;N=42) failed to exhibit a reduction in the response elicited by thepulse even when preceded by the pre-pulse (regardless of stimulusintensity). This reflects sensorimotor gating deficits common topatients afflicted with schizophrenia. Rats pretreated (60 min) withN-acetyl cysteine (30 mg/kg, po) failed to exhibit sensorimotor gating.Note direct delivery of N-acetyl cysteine into the brain reversesphencyclidine-induced deficits in sensorimotor gating, which isconsistent with clinical trials establishing the antipsychotic efficacyof this compound. Rats pretreated (60 min) with compounds synthesizedfrom scheme 1 (N=7-22/group), notably compounds 5a-D and 4a, exhibited asignificant difference relative to either rats receiving PCP alone (*,Fisher LSD, p<0.05) and/or N-acetylcysteine (N 30; 30 mg/kg; +, FisherLSD, p<0.05). Collectively, these data indicate the efficacy of thesecompounds and this synthesis scheme to generate novel antipsychoticsthat exceeds the potential of N-acetyl cysteine.

Example 7 Efficacy of Compounds from Scheme 2 Relative to N-acetylcysteine in Reversing Pcp-Induced Deficits in Sensorimotor Gating inRats

FIG. 7 is a bar graph illustrating inhibition of a startle response inresponse to a load stimulus (pulse) when preceded by a pre-pulsestimulus (2-15 db above background). Prepulse inhibition is a commonlyused paradigm to screen antipsychotic agents for use in treatingschizophrenia. The pre-pulse stimulus presented at 15 dB abovebackground reduced the startle response in saline controls (S; N=46)by >60% relative to the response elicited following exposure to thepulse only. Rats pretreated with phencyclidine only (P; 1.25 mg/kg, SC;N=42) failed to exhibit a reduction in the response elicited by thepulse even when preceded by the pre-pulse (regardless of stimulusintensity). This reflects sensorimotor gating deficits common topatients afflicted with schizophrenia. Rats pretreated (60 min) withN-acetyl cysteine (30 mg/kg, po) failed to exhibit sensorimotor gating.Note direct delivery of N-acetyl cysteine into the brain reversesphencyclidine-induced deficits in sensorimotor gating, which isconsistent with clinical trials establishing the antipsychotic efficacyof this compound. Rats pretreated (60 min) with compounds synthesizedfrom scheme 2 (N=7-14/group), notably compounds 5a and 7a, exhibited asignificant difference relative to either rats receiving PCP alone (*,Fisher LSD, p<0.05) and/or N-acetylcysteine (N 30; 30 mg/kg; +, FisherLSD, p<0.05). Collectively, these data indicate the efficacy of thesecompounds and this synthesis scheme to generate novel antipsychoticsthat exceeds the potential of N-acetyl cysteine.

Example 8 Efficacy of Compounds from Scheme 3 Relative to N-acetylcysteine in Reversing Pcp-Induced Deficits in Sensorimotor Gating inRats

FIG. 8 is a bar graph illustrating inhibition of a startle response inresponse to a load stimulus (pulse) when preceded by a pre-pulsestimulus (2-15 db above background). Prepulse inhibition is a commonlyused paradigm to screen antipsychotic agents for use in treatingschizophrenia. The pre-pulse stimulus presented at 15 dB abovebackground reduced the startle response in saline controls (S; N=46)by >60% relative to the response elicited following exposure to thepulse only. Rats pretreated with phencyclidine only (P; 1.25 mg/kg, SC;N=42) failed to exhibit a reduction in the response elicited by thepulse even when preceded by the pre-pulse (regardless of stimulusintensity). This reflects sensorimotor gating deficits common topatients afflicted with schizophrenia. Rats pretreated (60 min) withN-acetyl cysteine (30 mg/kg, po) failed to exhibit sensorimotor gating.Note direct delivery of N-acetyl cysteine into the brain reversesphencyclidine-induced deficits in sensorimotor gating, which isconsistent with clinical trials establishing the antipsychotic efficacyof this compound. Rats pretreated (60 min) with compounds synthesizedfrom scheme 3 (N=7/group), namely compounds 11-a-b and 11-a-d, exhibiteda significant difference relative to either rats receiving PCP alone (*,Fisher LSD, p<0.05) and/or N-acetylcysteine (N 30; 30 mg/kg; +, FisherLSD, p<0.05). Collectively, these data indicate the efficacy of thesecompounds and this synthesis scheme to generate novel antipsychoticsthat exceeds the potential of N-acetyl cysteine.

Example 9 Efficacy of Compounds from Scheme 4 Relative to N-acetylcysteine in Reversing Pcp-Induced Deficits in Sensorimotor Gating inRats

FIG. 9 is a bar graph illustrating inhibition of a startle response inresponse to a load stimulus (pulse) when preceded by a pre-pulsestimulus (2-15 db above background). Prepulse inhibition is a commonlyused paradigm to screen antipsychotic agents for use in treatingschizophrenia. The pre-pulse stimulus presented at 15 dB abovebackground reduced the startle response in saline controls (S; N=46)by >60% relative to the response elicited following exposure to thepulse only. Rats pretreated with phencyclidine only (P; 1.25 mg/kg, SC;N=42) failed to exhibit a reduction in the response elicited by thepulse even when preceded by the pre-pulse (regardless of stimulusintensity). This reflects sensorimotor gating deficits common topatients afflicted with schizophrenia. Rats pretreated (60 min) withN-acetyl cysteine (30 mg/kg, po) failed to exhibit sensorimotor gating.Note direct delivery of N-acetyl cysteine into the brain reversesphencyclidine-induced deficits in sensorimotor gating, which isconsistent with clinical trials establishing the antipsychotic efficacyof this compound. Rats pretreated (60 min) with compounds synthesizedfrom scheme 4 (N=7/group), namely the intermediate to compound 14a(Inter-14a) and compound 15f, exhibited a significant differencerelative to either rats receiving PCP alone (*, Fisher LSD, p<0.05)and/or N-acetylcysteine (N 30; 30 mg/kg; +, Fisher LSD, p<0.05).Collectively, these data indicate the efficacy of these compounds andthis synthesis scheme to generate novel antipsychotics that exceeds thepotential of N-acetyl cysteine.

Example 10 Efficacy of Compound from Scheme 5 Relative to N-acetylcysteine in Reversing Pcp-Induced Deficits in Sensorimotor Gating inRats

FIG. 10 is a bar graph illustrating inhibition of a startle response inresponse to a load stimulus (pulse) when preceded by a pre-pulsestimulus (2-15 db above background). Prepulse inhibition is a commonlyused paradigm to screen antipsychotic agents for use in treatingschizophrenia. The pre-pulse stimulus presented at 15 dB abovebackground reduced the startle response in saline controls (S; N=46)by >60% relative to the response elicited following exposure to thepulse only. Rats pretreated with phencyclidine only (P; 1.25 mg/kg, SC;N=42) failed to exhibit a reduction in the response elicited by thepulse even when preceded by the pre-pulse (regardless of stimulusintensity). This reflects sensorimotor gating deficits common topatients afflicted with schizophrenia. Rats pretreated (60 min) withN-acetyl cysteine (30 mg/kg, po) failed to exhibit sensorimotor gating.Note direct delivery of N-acetyl cysteine into the brain reversesphencyclidine-induced deficits in sensorimotor gating, which isconsistent with clinical trials establishing the antipsychotic efficacyof this compound. Rats pretreated (60 min) with a compound (18e)synthesized from scheme 5 (N=7) exhibited a significant differencerelative to either rats receiving N-acetylcysteine (N 30; 30 mg/kg; +,Fisher LSD, p<0.05). Collectively, these data indicate the efficacy ofthis compound and synthesis scheme to generate novel antipsychotics thatexceeds the potential of N-acetyl cysteine.

Example 11 Efficacy of Compound 5a-d (Scheme 1) as Novel AnticravingAgent

In addition to normalizing the function of the prefrontal cortex, asdemonstrated by the impact of the prodrugs on pcp-induced sensorimotorgating deficits, the anticraving potential of a drug can be demonstratedusing the extinction/reinstatement paradigm. In the present experiments,rats were implanted with indwelling jugular catheters with an externalport affixed slightly posterior to the rat's shoulder blades. Tubing isused to connect a syringe of cocaine to the external port of theindwelling catheter. Rats are then placed into standard operant chambers(Med Associates) and permitted to press a lever for an infusion ofcocaine (0.5 mg/kg/200 microL, IV). Once behavior is stable, rats arepermitted at least eleven 2-hr sessions to self-administer cocaine.Afterwards, the cocaine solution is replaced with saline in order toextinguish lever pressing. Once responding decreases to 10 or fewerlever presses/2 hr sessions for 3 out of 4 daily sessions, rats aretested for reinstatement (relapse). To do this, rats are placed into theoperant chamber and vehicle or a cysteine/cystine prodrug (1-60 mg/kg,p.o.; N=2-17) is administered. Afterwards, rats then receive aninjection of cocaine (10 mg/kg, IP). Responding is then assessed for 120min. Data depicted in FIG. 11 illustrate that N-acetyl cysteine (IP) iseffective in producing a significant reduction in cocaine-inducedreinstatement at the doses of 30 and 60 mg/kg (IP; * indicates asignificant decrease in responding relative to rats treated with 0 NAC,Fisher LSD). FIG. 12 demonstrates that N-acetylcysteine is lesseffective when given orally. Further, administration of 1 mg/kg ofCompound 5a-d (Scheme 1) was sufficient to block cocaine-inducedreinstatement, an effect that was comparable to 30 mg/kg NAC (*indicates a significant decrease in responding relative to rats treatedwith 0 NAC, Fisher LSD).

While this invention has been described in conjunction with the variousexemplary embodiments outlined above, various alternatives,modifications, variations, improvements and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent to those having at least ordinary skill in the art.Accordingly, the exemplary embodiments according to this invention, asset forth above, are intended to be illustrative, not limiting. Variouschanges may be made without departing from the spirit and scope of theinvention. All publications, patents and patent applications citedherein are hereby incorporated by reference in their entirety for allpurposes.

What is claimed is:

1. A cysteine prodrug having the structure:

wherein: R¹ and R² are independently selected from OH, ═O, or a branchedor straight chain C₁ to C₅ alkoxyl group, with the caveat that when ═Ois selected the nitrogen atom adjacent the carbonyl group thusly formedbears a H and a single bond joins the adjacent nitrogen to said carbonylgroup; R³ is H, a branched or straight chain C₁ to C₅ alkyl, anitrobenzenesulfonyl, a trityl, an aryl thio, an aryl, an alkylthio, anacyl, a benzoyl, a thio acyl, a thio benzoyl, or a benzyl group; and R⁴is selected from the side chain groups of the natural L-amino acids cys,gly, phe, pro, val, ser, arg, asp, asn, glu, gln, ala, his, ile, leu,lys, met, thr, trp, tyr, or D-isomers thereof, with the caveat that whenR⁴ is the side chain group of the natural L-amino acid gly, R¹ and R²are not both selected to be ═O; or a cystine dimer of said prodrughaving the structure:

wherein: R¹, R^(2,) R⁵ and R⁶ are independently selected from OH, ═O, ora branched or straight chain C₁ to C₅ alkoxyl group, with the caveatthat when ═O is selected the nitrogen atom adjacent the carbonyl groupthusly formed bears a H and a single bond joins the adjacent nitrogen tosaid carbonyl group; and R⁴ and R⁷ are independently selected from theside chain groups of the natural L-amino acids cys, gly, phe, pro, val,ser, arg, asp, asn, glu, gln, ala, his, ile, leu, lys, met, thr, trp,tyr, or D-isomers thereof, with the caveat that when R⁴ and R⁷ are boththe side chain group of the natural L-amino acid gly, R¹, R^(2,) R⁵ andR⁶ shall not all be selected to be ═O.
 2. The cysteine prodrug accordingto claim 1, wherein said cysteine prodrug has the structure:


3. The cysteine prodrug according to claim 1, wherein said cysteineprodrug is in the form of the cystine dimer having the structure:


4. The cysteine prodrug according to claim 1, wherein said cysteineprodrug in the form of the cystine dimer includes identical R⁴ and R⁷groups.
 5. The cysteine prodrug according to claim 1, wherein saidcysteine prodrug in the form of the cystine dimer includes non-identicalR⁴ and R⁷ groups.
 6. The cysteine prodrug according to claim 1, whereinsaid cysteine prodrug or cystine dimer thereof includes at least one R⁴and R⁷ group that is a cys, said cys further protected by a branched orstraight chain C₁ to C₅ alkyl, a nitrobenzenesulfonyl, a trityl, an arylthio, an aryl, an alkylthio, an acyl, a benzoyl, a thio acyl, a thiobenzoyl, or a benzyl group.
 7. A method of reducing schizophrenia in asubject comprising administering to said subject an effective amount ofa cysteine prodrug or cystine dimer thereof according to claim 1,whereby schizophrenia is reduced in said subject.
 8. The methodaccording to claim 7, wherein the step of administering to said subjectis accomplished by oral delivery.
 9. A pharmaceutical compositioncomprising a cysteine prodrug or cystine dimer thereof according toclaim 1 and a pharmaceutically-acceptable carrier.
 10. A method ofreducing drug craving in a subject comprising administering to saidsubject an effective amount of a cysteine prodrug or cystine dimerthereof according to claim 1, whereby drug craving is reduced in saidsubject.
 11. The method according to claim 10, wherein the step ofadministering to said subject is accomplished by oral delivery.
 12. Aprotected cysteine analog having the structure:

or a cystine dimer of said protected cysteine analog having thestructure:

wherein R¹ through R⁶ are independently selected from a branched orstraight chain C₁ to C₅ alkyl, a phenyl, or a benzyl group.
 13. Theprotected cysteine analog according to claim 12, wherein said protectedcysteine analog has the structure:


14. The protected cysteine analog according to claim 12, wherein saidprotected cysteine analog is in the form of the cystine dimer having thestructure:


15. A method of reducing schizophrenia in a subject comprisingadministering to said subject an effective amount of a protectedcysteine analog or cystine dimer thereof according to claim 12, wherebyschizophrenia is reduced in said subject.
 16. The method according toclaim 15, wherein the step of administering to said subject isaccomplished by oral delivery.
 17. A pharmaceutical compositioncomprising a protected cysteine analog or cystine dimer thereofaccording to claim 12 and a pharmaceutically acceptable carrier.